The present invention relates to a process for preparing a polyimide film by a casting process with a T-die in extrusion. Specifically, the present invention relates to a process for preparing a polyimide film, wherein bubbles are prevented from being mixed into a resin solution (referred to as xe2x80x9ccurtainxe2x80x9d in the present specification) which is extruded from a die in a T-die method, and unevenness in thickness is controlled, and wherein mechanical properties, especially tensile strength, of the polyimide film to be obtained are prevented from decreasing.
Among plastic materials, polyimides have excellent properties in heat resistance, insulating properties, solvent resistance and low temperature resistance, and are used as a material for electrical and electronic parts. Specifically, they are used for flexible print circuit, base films in TAB carrier tape, coating materials of electric cables for aircraft and the like, base films in magnetic recording tape, coating materials of superconducting coil wire, and the like. A polyimide film suitable for each purpose is selected appropriately for the above various purposes.
In accordance with miniaturization and thin filming in electrical and electronic parts, line thinning of circuit is in progress. Dimensional change of materials during use may cause accidents such as disconnection and short-circuit in thinly lined circuit structure. Therefore, highly accurate dimensional stability is required for materials used for electrical and electronic parts.
Meanwhile, as shown in FIG. 1, a polyimide film is prepared as follows. A polyimide precursor, i.e., a composition of poly(amic acid) solution is mixed with a chemically-imidizing catalyst in an extrusion machine 2; the mixture is spread from the extrusion machine 2 in the cross direction; and then the mixture is extruded continuously into a smooth, thin film through a die lip 6, namely, a narrow slit space set on a slit die 4 on an endless belt 10. Then, in the progress of imidizing the above composition of poly(amic acid) solution, a resin film 8 is hardened by drying and cooling to such a degree that the resin achieves self supportability; and further, the hardened resin is heat-treated to prepare a polyimide film.
However, in the above process for preparing a polyimide film, an acid dianhydride component and a diamine component are normally mixed in almost equimolar amount, specifically in a component ratio ranging from about 1:0.095 to 1:1.005 as raw materials of a poly(amic acid) composition to obtain the poly(amic acid) composition. The poly(amic acid) composition is usually a viscous liquid whose viscosity is adjusted to at least 2,500 poise. The viscosity of a liquid or slurry composition of a resin solution is usually in the range of 500 to 2,000 poise when it is extruded from the slit die after mixing the above poly(amic acid) composition and the chemically-imidizing agent.
Also, a polyimide film is obtained by casting a poly(aimic acid) composition which is a polyimide precursor according to a casting method with a T-die. Thereafter, imidization is completed through film forming, heating and drying, to obtain a polyimide film. During the casting step, if imidizing reaction of the poly(amic acid) composition proceeds rapidly, a resin film is partially imidized, leading to problems with generation of gel defect parts in a film as well as formation of coating streaks since the slit die is clogged with the gel generated by the partial imidization of the poly(amic acid)composition. Therefore, in order to control the imidizing reaction of the poly(amic acid)composition, the composition of poly(amic acid) solution is generally cooled to at most 0xc2x0 C. Viscosity of the composition of poly(amic acid) solution tends to become considerably high due to the cooling.
A composition of a resin solution has elasticity when it has a relatively high viscosity such as the above, namely, 500 to 2,000 poise. Therefore, as shown in FIG. 2, curtain 22 which is a fluid composition of a resin solution extruded from the die lip 6 is drawn to machine direction as the speed of an endless belt 210 accelerates. Once the curtain 22 is drawn to the machine direction, an angle xcex8 formed when the curtain 22 reaches the endless belt 10 becomes smaller. Accordingly, when the curtain 22 reaches the surface of the endless belt 10, air is easily included between the curtain 22 and the endless belt.
As a result, air can be included between the prepared resin film 8 and the endless belt 10, and foamed projection parts having large and small sizes may remain on the surface of the resin film 8. The inclusion of bubbles causes to deteriorate surface properties of resin film remarkably. For example, in the drying step of the above resin film, the film thickness of the projected part becomes thin, the included air is expanded to break some part of the resin film, and defective parts are generated.
Further, since the above-mentioned high viscosity curtain has stronger elasticity and larger adhesive strength to the belt than low viscosity curtain, it is drawn to the machine direction depending on the movement of the belt. However, when the curtain is drawn to the machine direction for at least a certain distance, the reaching point of curtain varies periodically since reverse force is generated against the machine direction due to the elasticity of the resin film. The periodic variance causes variation in thickness of the resin film to be prepared, and therefore periodic unevenness in thickness is generated in the machine direction, resulting in the problem that such unevenness appears as striping on the surface of the film as the final product.
In response to this problem, Japanese Unexamined Patent Publication 198157/1999 discloses a process for casting a film in which viscosity of a composition of a resin solution in a die is lowered. The process aims at preventing bubble inclusion at casting a resin film, improving uneven thickness and promoting production efficiency of the film even in a process for casting a film at high speed. As a process for lowing viscosity of a composition of a resin solution in a die, there are disclosed a process for decreasing polymerization degree of a composition of a resin solution and a process for increasing the ratio of the solvent in the composition of a resin solution.
However, a polyimide film prepared according to the process for lowering the polymerization degree disclosed in Japanese Unexamined Patent Publication 198157/1999 had a problem that mechanical properties of the film are significantly reduced compared with a polyimide film prepared from equimolar diamine component and tetracarboxylic dianhydride component. Also, in the process for increasing the ratio of the solvent in the composition of a resin solution disclosed in Japanese Unexamined Patent Publication 198157/1999, temperature of the belt needs to be raised high in order to dry the resin on the endless belt to such a degree that self supportability is achieved, and in consequence, mechanical properties of the polyimide film to be obtained is lowered.
As mentioned above, in the process for casting a film in which inclusion of bubbles are prevented at casting a resin film and uneven thickness is improved, there is a problem that mechanical properties of the polyimide film to be obtained are lowered significantly. The lowering of the mechanical properties causes to disturb stable production due to generation of waviness made by film extension in the process of producing flexible printcircuit, base films in TAB carrier tape, coating materials of electric cables for aircraft and the like, base films in magnetic recording tape, coating materials of superconducting coil wire, and the like. Also, mechanical properties of products made therefrom are lowered even to make the products unreliable.
The present invention is to provide a process for preparing a polyimide film wherein inclusion of bubbles are prevented at resin film casting, and uneven thickness is improved especially in such a cast film forming process for preparing polyimide film at high speed as the above without the lowering of mechanical properties as seen in the process for casting a film disclosed in Japanese Unexamined Patent Publication 198157/1999 at the same time.
The present invention has been carried out to solve the above problems. The process for preparing a polyimide film of the present invention relates to a process for preparing a polyimide film by extruding and casting a composition of a resin solution containing a poly(amic acid) varnish, which comprises a step for preparing the poly(amic acid) varnish which is a varnish having low viscosity by polymerizing a tetracarboxylic dianhydride component with a diamine component in a molar ratio of 1:1.01 to 1:1.05, or 1:0.95 to 1:0.99, and a step for preparing the composition of a resin solution by adding, to the poly(amic acid) varnish, a dehydrating agent in a molar ratio of at least 1 time and a chemically-imidizing catalyst in a molar ratio of at least half time based on 1 mole of the amic acid of the poly(amic acid) varnish.
It is preferable that viscosity of the poly(amic acid) varnish is at most 2,000 poise at 20xc2x0 C.
It is also preferable that viscosity of the poly(amic acid) varnish is at most 1,500 poise at 20xc2x0 C.
It is preferable that viscosity of the poly(amic acid) varnish is 100 to 1,500 poise at 20xc2x0 C. 
It is preferable that the composition of a resin solution is obtained by adding the dehydrating agent of at least 1.2 to 5 times and the chemically-imidizing catalyst of at least 0.6 to 2.0 times in molar ratio based on 1 mole of the amic acid of the poly(amic acid) varnish.
It is preferable that the chemically-imidizing catalyst is a tertiary amine.